I had the Neutron IVs. Great little minis. Against my Spendor S3/5R, Xeros and Dyn DM 2/6, I'd say on the scale of added warmth , it is '0' i.e., it is pretty neutral but can sound lean with the wrong partners. You're likely to need a powerful and beefy sounding ss amp (>70w) to bring out body in its mids, and efface its 'tight' voicing. This tautness has some similairy to the sealed-box ATC SCM11. Both can sound unyielding and dark with hard-sounding ss amps.
I'm a bit of an Arcam fanboy these days, but I not sure the Solo's modest wattage is ideally suited for the Neutrons. I'd prolly prefer to match the Solo with the Xeros or Dyn DM standmount.
IMO, the Xero is more liquid-sounding than the Neutron IV, while an equivalent Dyn mini (DM 2/6 or X12, I last heard) is richer and fuller albeit less neutral.
The Neutron IV offer insightful detail into voices and has an exceptionally clear and detailed treble, the Scanspeak.